Control of the read/write head position in a disk drive relative to track centerline on the disk is necessary to insure accuracy of disk drive read and write operations. Over the years, many types of servo systems have been devised to detect and correct the alignment between the read/write head and the track centerline. One type of disk drive servo system is known as a sampled servo system. In such a system, which is characteristically closed-loop or track-following, servo information in the form of magnetic burst patterns is recorded in one or more servo sectors on each track on the surface of the disk. This servo information is read by the read/write head during servo operations at each servo sector and is used to generate position error signals as a function of the misalignment between the head and the disk track centerline. The position error signals are input through a microprocessor which in turn performs appropriate calculations with the position error signals and outputs servo compensation signals which control the disk drive head positioning mechanism to place the read/write heads over track centerline.
One well known type of sampled servo system involves recording groups or bursts of magnetic transitions radially displaced from each other in the servo sector. These bursts are typically designated as the "A" burst and "B" burst. Due to the radial displacement of the "A" burst relative to the "B" burst in a given servo sector, the "A" and "B" bursts are displaced on either side of the track centerline. When the head is positioned exactly over track centerline approximately one-half of the "A" burst will be read followed by one-half of the "B" burst in a time displaced fashion. As the head moves off track, the amplitude of one burst decreases while the amplitude of the other burst increases depending on the direction of misalignment. In this manner, a position error signal can be derived from the relative amplitudes of the bursts by rectifying and peak detecting the readout from the head as it passes over the "A" and "B" bursts, and determining the difference in amplitude between the bursts.